Phenomenon has a specialized meaning in the philosophy of Immanuel Kant who opposed the term 'Phenomenon' to 'Noumenon'. Phenomena constitute the world as we experience it, as opposed to the world as it exists independently of our experiences (thing-in-themselves). Humans cannot, according to Kant, know things-in-themselves, only things as we experience them. Thus philosophy should concern itself with understanding the process of experience itself.

Kant's account of phenomena has also been understood as influential in the development of psychodynamic[?] models of Psychology, and of theories concerning the ways in which the brain, mind and external world interact.

In general, apart from its specialized use as a term in philosophy, phenomenon stands for any observable event. Phenomena make up the raw data of science. Phenomena are often exploited by technology.

It is possible to list the phenomena which are relevant to almost any field of endeavor, for example, in the case of optics and light one can list observable phenomena under the topic
optical phenomenon.